High affinity, stereospecific recognition sites (receptors) for neurotransmitters, neuromodulators, and many clinically useful drugs have been identified in both peripheral tissues and the central nervous system. The interaction of a neurotransmitter, neuromodulator or drug with a specific recognition site initiates a series of events (for example, the opening of an ion channel or activation of an enzyme) resulting in either a physiological/behavioral response (in the case of a neurotransmitter or neuromodulator)or pharmacological effect (in the case of a drug) . Furthermore, the presence of recognition sites for synthetic compounds suggests that endogenous substances may also be present that mimic (or antagonize) the effects of exogenously applied substances. Studies are in progress to characterize "recognition- effector" systems, to link novel recognition sites to effector systems, and to relate these systems to both physiological and pathological processes. Systems under study include: a) the benzodiazepine/GABA receptor chloride ionophore complex; b) the glycine-gated chloride ionophore; c) "peripheral" benzodiazepine receptors (in both peripheral tissues and the central nervous system) and d) glycine receptors linked to NMDA-gated cation channels.